Conventional electronic assembly involves placing a plurality of electronic components on wiring or printed circuit boards which include one or more metal interconnect levels having board land pads on their surface. The electronic components are temporarily immobilized by contact to pre-deposited lead (Pb)-based solder paste on the board. The lead-based solder paste is then reflowed at a temperature of about 150 to 180° C. to provide a permanent immobilization and electrical contact between leads, pads or other bonding features on the electronic components and the board land pads.
On Jul. 1, 2006 the European Union Waste Electrical and Electronic Equipment Directive (WEEE) and Restriction of Hazardous Substances Directive (ROHS) came into effect prohibiting the intentional addition of lead and thus lead-based solder to most consumer electronics produced in the European Union. California recently adopted a ROHS law and China has a version as well. Manufacturers in the U.S. may receive tax benefits by reducing the use of lead-based solder.
Military industry can still use lead-based solder. However, military applications only make up about 1% of electronic usage in the world. Because such military usage is so small, electronic assembly suppliers have chosen to ignore the military needs resulting in lead-free or pure tin finished electronic assembled parts for both military and commercial applications. However, because of relatively high temperature processing needed for lead-free or pure tin solder, there is a risk of component damage or tin whisker formation from pure tin which can result in shorts or opens from thermal shock and thermal cycle fatigue-based circuit board failures.